In industrial solvent extraction processes, a viscous material often accumulates in liquid phase decanting ponds. This material is a close mixture of organic and aqueous solutions together with extremely fine solid particles which can be either organic or inorganic in nature. In some cases, the mixture also contains air distributed as a fine dispersion of bubbles.
The above-described mixture is present as an emulsified system of small drops of organic material suspended in an aqueous matrix, with the solid particles at the organic/aqueous interphase. Extraction plant operators commonly refer to these materials as "crud" "gunk" "grungies" "lumps" or "lees" (hereinafter collectively referred to as "crud" or "emulsions") and try to minimize their occurrence since their presence can alter the behavior of the system during phase separation. In particular, these emulsions can affect dispersion band widths in decanters, the stability of phase dispersion in mixers, and the quantity of the residue of one liquid phase, i.e., present as micro-drops, found in the other liquid phase.
In practice, these emulsions result in losses of expensive organic reagent as well as in contamination of the electrowinning electrolyte or other subsequent process. The contamination may be due to the presence of micro-drop residues of the original or degraded organic, or residues in the advance solution at a subsequent stage (e.g., residues of organic in aqueous, i.e., "O/A"). Contamination can also be due to an aqueous solution from leeching circuit which is transferred to the electrolyte by way of the organic phase as residues of aqueous in organic ("A/O"). The presence of such impurities in the electrowinning electrolyte can deteriorate cathodic quality, contribute to increased corrosion of anodes, and decrease the current efficiency of the electrolytic process, thus consequently increasing operating costs. In some instances, moreover, these residues can compromise the quality of the final product.
Crud is produced due to the emulsifying action exerted by fine and colloidal solids contained in aqueous solutions that circulate through the extraction circuit. For the most part, these solids are hydrophilic and thus are located in the organic/aqueous interphase under conditions of reduced free interfacial energy which occur when the surface of the solids exposed to the organic phases is minimal. The steric configuration of the particles curves the convex interphase toward the organic solution, inducing the formation of small drops of organic in a continuous aqueous medium, thus giving rise to crud formation.
Accordingly, if it were possible to eliminate the fine solids from the aqueous solutions used in solvent extraction processes, the emulsions known as "crud" would not form in the decanters. In practice, however, it is unlikely that the solids can be completely eliminated. Fine particles enter the system suspended in strong solutions when the solids have been collected by the natural passage of leeching material through material mineralized in pile leeching, dumps, washing troughs or other types of beds. Further, fine particles may also exist as part of the matrix in the form of clays or other alteration products of the host rock, or may form by the action of the leeching agent, as occurs with colloidal silica generated by chemical dissolution of silicates. These solids may also be drawn from the conduits and storage areas for the leething solutions, particularly during very rainy periods. Such solids additionally result from an incomplete solid/liquid separation in agitated leeching processes. Solids can also be produced by post-precipitation of hydrolyzable species or solid products of anode corrosion, environmental dust, fungus, or even bacteria.
Due to the innumerable ways that hydrophilic fine particles can be introduced into an extraction system, therefore, formation of crud always occurs to some extent. It is therefore necessary to remove the crud from the decanters, which removal thus constitutes a normal periodic operating step. One method for removing such crud is to decant the emulsion for prolonged periods, after which a portion of the trapped organic can be recovered since emulsions are thermodynamically unstable systems and thus their coalescence is simply a matter of kinetics. Having a portion of the inventory of organic material outside the circuit, however, can significantly add to the financial costs associated with the extraction process. Thus, it is often necessary to perform additional process steps to recover the organic material contained in the emulsions.
A common method for breaking such emulsions, particularly in the field of copper technology, comprises centrifuging the emulsions using high-cost continuous centrifugation units. The capacity and operational efficiency of such centrifuges are reduced, however, by their limited availability as well as high cleanliness and maintenance requirements. Thus, although many extraction plants have centrifuges, they are seldom used.
An alternative process used in some plants comprises spreading water vapor over the surface of emulsions contained in a storage pond until the temperature of the system is raised between 43.degree. and 49.degree. C. This enhances the coalescence of the system, making it possible for a centrifuge to deal with a smaller quantity of the emulsions. However, this method generates compacted emulsions that, compared to the original, i.e., "primary" emulsions, can be significantly more difficult to break. In addition, local overheating due to insufficient temperature control can induce catalytic degradation of the extractant.
In other instances, e.g., where the emulsions accumulate in a re-extraction stage, which is a frequent situation in certain plants, the emulsions are periodically pumped to the discharge point of a mixer used in the last stage of extraction, entering underneath the dispersion band into the decanter, where they coalesce. This technique has very limited application, however, since it is only effective when the emulsions have very low stability. Moreover, the remaining emulsions act as emulsifying agents and can invert the continuous phases in those mixers that operate in continuous organic. This process can also increase losses of organic in the aqueous phase exiting the circuit.
Another process of practical interest is the mechanical breaking of emulsions as described, for example, in Chilean Patent No. 30,817. This method comprises withdrawing the emulsions from continuous decanters and emptying them into an agitation pond; adding a volume of an organic phase to the emulsions which is miscible with the organic part of the emulsions and immiscible with its aqueous part; agitating the phases so that during the mixing, the continuous phase is the organic phase; decanting the organic phase and reintegrating it into the extraction circuit. This process is efficient in many cases. However, even after the emulsions are broken, residual or secondary emulsions remain which may contain between 15 and 20% of the original organic material. These residual or secondary emulsions are normally sent to a drainage pond where a portion of the remaining organic is recovered by simple decanting over a prolonged period. As an alternative, if a centrifuge is available, the secondary emulsions can be treated thereby as described above.